1. Technical Field
The present invention relates to integrated services digital network communications, and more particularly relates to an apparatus and method for controlling intralayer and interlayer entity communications to interface a private exchange to an integrated services digital network (ISDN).
2. Related Art
Generally, a conventional telephone network or data network is a dedicated service network intended to provide only specific communication service such as telephone service or data service to terminal subscribers. Typically, key phone systems and other private exchanges are connected to a public switched telephone network (PSTN) to provide voice communication service. In contrast to the conventional telephone network, an integrated services digital network (ISDN) offers a variety of new features including multimedia communication service such as voice, high speed data and image communication services and other additional non-voice communication services via network interfaces on the basis of digitization of the telephone network. Contemporary ISDN communication systems including private exchanges are disclosed, for example, in U.S. Pat. No. 4,998,248 for Integrated Services Digital Network (ISDN) System And Multimedia Data Connection Method In Same System issued to Matsuzaki, U.S. Pat. No. 5,305,312 for Apparatus For Interfacing Analog Telephones And Digital Data Terminals To An ISDN Line issued to Fornek et al., U.S. Pat. No. 5,450,396 for Communication System And A Private Branch Exchange To Be Used In Such A Communication System issued to Havermans, and U.S. Pat. No. 5,621,731 for Private Exchange For ISDN issued to Dale et al.
As the ISDN communication systems become widely in use, the private exchange and its terminal stations connected to the ISDN must effectively accommodate multimedia communication services and a variety of other additional services through the ISDN. ISDN interfaces must be provided in the private exchange on the basis of network interface standards. Exemplary ISDN interfaces for an ISDN communication system are disclosed in U.S. Pat. No. 5,305,313 for Electronic Switching System For Use In Connection To An ISDN And Method Of Setting Communication Disconnection Reasons issued to Katoh, U.S. Pat. No. 5,450,486 for Telecommunication System And A Linecard issued to Maas et al., U.S. Pat. No. 5,481,598 for Subscriber Terminal For ISDN Networks issued to Bergler et al., U.S. Pat. No. 5,557,663 for Multi-Media Communication System With Integrated And Coherent Audio-Video User Interface Allowing Flexible Image Input issued to Huang et al., and U.S. Pat. No. 5,566,301 for ISDN Audiovisual Teleservices Interface Subsystem issued to Koz et al. While these ISDN interfaces are implemented in accordance with network interface standards, I have noted that further improvement for interfacing a private exchange to an ISDN can still be contemplated.